So we want get "a" by its self. To do this we need to get rid of the denominator of 1.6. So we multiply BOTH SIDES by 1.6 because what ever you do to one side you must do to the other.
-7.0625=a/1.6
1.6*(-7.0625)=(a/1.6)*1.6
The reason we multiply by 1.6 is because it cancels out the denominator of 1.6 on the "a" side because 1.6/1.6= 1
You are now left with.
1.6*(-7.0625)=a
-11.3=a